This invention relates to a method of allowing increased engine torque at certain transmission ratios.
In the prior art, transmissions and engines are becoming increasingly complex. In particular, transmissions for heavy vehicles such as trucks include a number of speed ratios. The speed ratios are provided by shifting gears into and out of engagement to selectively apply gear reduction to a rotational speed supplied by the engine to the transmission.
Transmissions can be one of two general types. Some transmissions have an overdrive capability such that the speed out of the transmission may actually be higher than the speed from the engine. However, it has been found that such overdrive transmissions are less energy efficient at the higher speed ratios than a second type, known as a direct drive transmission.
In a direct drive transmission, the highest speed ratio is a direct drive from the transmission input shaft to the transmission output shaft. The transmission input shaft is connected to the crankshaft of the engine. In one well-known type of direct drive transmission, ten speed ratios are available. By selectively connecting the input shaft to a transmission main shaft, and the transmission main shaft directly to the transmission output shaft, a one-to-one direct drive is provided. This supplies the highest speed ratio from the transmission.
The transmission also allows nine lower speed ratios by selectively engaging other gears. Essentially, the direct connection between the transmission input shaft and the transmission main shaft is broken. Instead, rotation is transmitted through counter shafts and associated gears that, in combination, result in a speed reduction to the transmission main shaft. This type of transmission is well-known and very successful.
Transmissions are rated for a maximum torque that can be transmitted through the transmission and to the transmission output shaft. Typically, a control module for the engine is provided with information on a maximum torque level. The transmission that is associated with the engine must be rated for transmitting that maximum torque level. The engine control module thus limits the torque from the engine that is transmitted to the transmission. Recently, engine manufacturers and vehicle designers have begun to request higher torque loads be transmitted through the transmission under certain conditions. It has been difficult for the transmission designer to accommodate these higher torque loads.